The invention relates to an improved process for the photolithographic reproduction of visual subject matter having a continuous tone and gradations of tonal value such as wash drawings, photographs and paintings. The invention further provides a flexible contact screen for use in the improved process and a method for producing the screen. More specifically, the present invention defines improvements in the process disclosed in the Applicant's own prior patent, U.S. Pat. No. 3,581,660 granted June 1, 1971, hereinafter referred to as the Rapoport et al. patent.
In the Rapoport et al. patent, the Applicants describe a process for the photolithographic reproduction of visual subject matter including dual printing steps, each done with a printing plate made by well known photolithographic techniques from a master and secondary negative produced by exposing the original subject matter to the light source through a patterned transparent screen. The screen used to make the master negative is a conventional half tone screen while the screen used for the secondary negative is a random pattern contact screen. The secondary negative is exposed to the original copy for a longer time interval, under the same lighting conditions to produce a printing plate that when printed in registry with the master printing plate, results in a final reproduction having extraordinary detail in the shadow areas of the original copy.
Other prior art patents disclosing lithographic printing processes including dual printing steps are the Gast U.S. Pat. No. 547,780; McIntosh U.S. Pat. No. 871,234; Weyl U.S. Pat. No. 809,157 and Cooke U.S. Pat. No. 3,130,669. The Gast and Weyl processes are for three color printing using a "stipple" screen for at least one of the printing steps to avoid what is known in the art as the "Moire" effect. In the McIntosh process a first printing step is performed with a printing plate produced from a half tone negative. According to the McIntosh teaching, the first print is overprinted with a printing plate made from a screen negative, i.e., a negative made without the interposition of a screen between the film and the original copy during exposure. The Cooke patent describes a complex procedure for making "screenless" halftone negatives by employing a granular surfaced screen interposed between a treated and bleached continuous tone transparency of the original subject matter and a light source during exposure.
The prior art is replete with random pattern, stipple, grannular, and other types of irregularly surfaced or textured lithographic screens, as distinguished from geometric screens containing a geometric pattern such as the well known square ruled half tone screen and the dot half tone screen such as that described in the Yule U.S. Pat. No. 2,292,313. In addition to the screens disclosed in the aforementioned patents, screens of this type can be found in the following patents, among others: Pearson U.S. Pat. No. 1,945,865; Ewald U.S. Pat. No. 1,710,303; Zoller U.S. Pat. No. 2,082,475; Eckardt U.S. Pat. No. 2,130,735; Morris U.S. Pat. No. 1,161,824 and Great Britain patent specification No. 315,681. The published literature also contains teachings of irregular pattern screens of the types described in the foregoing patents.
Typically, the irregular pattern screens described in the prior art are designed to be placed between a light source and the film used to make the printing plate negative with a definite physical separation between the surface of the screen and the film. Screens of this type are usually rigid materials such as glass or plastic plates having a surface pattern either physically engraved or coated thereon and should be distinguished from flexible contact screens which are designed to be in physical contact with the printing plate negative during exposure to the original copy. Contact screens are typically a flexible photographic transparency of a physically engraved transparent plate; although rigid, physically engraved, contact screens are known in the art.
Of the prior art patents mentioned, only the Applicants' own Rapoport et al. patent discloses a flexible transparent random pattern contact screen. The contact screen of the Rapoport et al. patent is described as a negative phototransparency having a fine irregular pattern, including randomly disposed irregularly shaped areas.
It is a primary objective of the present invention to define specific improvements in the process described in the Applicant's prior Rapoport et al. patent.
It is a further objective of the invention to provide an improved contact screen for use in the improved process.
A still further objective of the invention is to provide a process for making the improved contact screen.